Radioactive thickness gauge



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United States Patent Ofitice Reissued Mar. 19, 1963 1 25,353 RADIOACTEVETHICKNESS GAUGE George 13. Foster, Worthington, and Walter H. Canter,

Jr., Coiumhus, Ohio, assignors to Industrial Nucleouics Corporation,Coiumbns, Ghio, a corporation of Ohio Original No. 2,933,606, dated Apr.19, 1960, Ser. No.

434,786, June 7, 1954. Application for reissue Nov. 6,

1961, Ser. No. 154,093

18 Claims. (Cl. 250-33.3)

Matter enclosed in heavy brackets [1 appears in the original patent butforms no part of this reissue specification; matter printed in italicsindicates the additions made by reissue.

The present invention relates generally to devices employingelectromagnetic radiation and more particularly to devices and methodsfor employing electromagnetic radiation produced by the passage of betarays through a suitable target.

There is a need for sources of electromagnetic radiations (X-rays andgamma-rays) having average energies within the range of .05 to 0.50 mev.for radiation gauges such as are used for determining the thickness,Weight, density, profile, etc. of various materials and products.Electromagnetic radiation within this range is available from commercialX-ray equipment and gamma emitting radioisotopes; however, each of thesesources has considerable disadvantages. Commercial X-ray equipment isavailable which can produce X-rays having average energies within therange of about .02 to .10 mev. X-rays within this energy range have onlya limited utility in radiation gauges because of their limitedpenetrating power. Commercial X-ray equipment is expensive, bulky,complicated to operate, and requires a certain amount of servicing. Inaddition, the intensity of the radiation produced by commercial X-rayequipment is less stable than the intensity of radiation emitted bygamma emitting radioisotopes. Practical radioisotopes are availablewhich emit gamma rays having average energies from about .50 to 1.5 mev.However, the manufacturing, handling, and storage of the source capsulscontaining such gamma ray emitters requires extreme precautions andexpensive facilities.

The main object of the present invention is to provide a source ofelectromagnetic radiation for use in radiation gauges which is free ofthe disadvantages which are attendant with commercial X-ray equipmentand gamma sources (radioisotopes).

Another object of the invention is to provide a method of and means forproducing electromagnetic radiation having an average energy between .05and .50 mev., the average energy being capable of preselection andvariation at will, depending upon the particular use to be made of theradiation.

Further objects of the invention are: to provide a radiation gaugewherein electromagnetic radiation emitted as a result of the passage ofbeta rays through a suitable target is directed at the material to bemeasured; and to provide a radiation gauge such as a thickness gaugewherein the material to be measured is placed between a beta source anda detector, a target of sufiicient thickness being positioned betweenthe beta source and the material being measured so that most of the rayswhich reach the material being measured comprise electromagneticradiation produced as a result of the deceleration or retardation of thebeta rays by the electrostatic field of nuclei of the target material.

Further objects, advantages, and features of the invention will becomeapparent from the following description of a preferred embodimentthereof, taken together with the accompanying drawings.

In the drawings:

given by the following formula.

FIGURE 1 is a schematic representation of a radiation gauge, such as athickness gauge, employing the present invention;

FIGURE 2 is a calibration curve using the apparatus of FIGURE 1 which isobtained by comparing ionization currents due to various thicknesses ofa material with the ionization current without the material;

FIGURE 3 is a graph showing the energy distribution of emitted photonsproduced in an aluminum target for various energies of the originatingelectrons;

FIGURE 4 is a graph showing the effect of target thickness, in thisgraph the thickness of an aluminum absorber required to reduce theionization current in an ionization chamber resulting from radiationpassing through or emitted from the target to one-half its value isplotted against thicknesses of aluminum target;

FIGURE 5 is a graph showing the effect of the target material, in thisgraph twice the thickness of absorber material required to reduce theionization current to onehalf its value is plotted against the atomicnumber of the absorber material for various target materials; and

FIGURE 6 is a graph showing the efiect of the filling gas on ionizationchamber response, in this graph percent response is plotted against theenergy of the radiation entering the chamber for various types offilling gas.

FIG. 7 is a schematic representation of a recording system coupled intothe measuring system of FIG. 1.

When charged particles such as electrons pass through matter, they loseenergy by inelastic collisions wherein a portion of their energy istransferred by electromagnetic interactions to the electrons of theatoms with which they collide, and by radiative collisions wherein theparticles are decelerated in the fields of the nuclei which they passwith a resultant loss of energy and emission of electromagneticradiation.

When electrons pass through matter, they lose energy by electromagneticinteractions which raise the electrons of the matter to excited energystates. The electrons may be raised to an excited bound state, or to afree state. In either case, the increment of energy given to theelectrons is taken from the kinetic energy of the incident electrons. Inthe following, the term ionization will refer to both degrees ofexcitation, that is, to either an excited bound state or to a freestate. The ionization loss of electrons as they pass through matter is(See Montgomery, Cosmic Ray Physics, p. 30.)

dX ion.

is the ionization loss in ev. per gum/cm. of an absorber; fl=v/c is theratio of the speed of the electron to the speed of light; N =6.02 10atoms/ gram atom (Avogadros number); Z is the atomic number of theabsorber; A is the atomic weight of the absorber;

(the classical radius of the electron); m c =0.5 109x 10 ev. is the restenergy of the electron; and I is the mean energy of excitation of theelectrons of the absorber. This formula shows that the ionization lossby electrons is proportional to the atomic number of the absorber andinversely proportional to the square of the velocity of the incidentelectrons.

The loss of energy of electrons resulting in electro magnetic radiationas they pass through matter is caused by the deflection of the incidentelectrons in the fields of the nuclei of the matter traversed. Thisradiation is known as [Bramsstrahlung] bremsstrahlung (literally braking[breaking] radiation, or radiation by collision). One may consider thisprocess as a transition from an initial state wherein the incidentelectrons have positive energies and the radiation field contains zerophotons to states where the electrons have smaller energies and theradiation field contains photons of varying energies. The radiative lossof electrons is given by the following formula. (See Fermi, NuclearPhysics, p. 47.)

dB NE 183 T)rad. 1?? T In Z-TN where dX red.

From this formula, one can see that free electrons which have arelatively low energy and/ or which pass through matter of low atomicnumber, lose energy mainly through ionization, whereas free electronswhich have a high energy and/or which pass through matter having a highatomic number, lose energy mainly through radiation.

In the case of beta rays (electrons) emitted by a radioisotope (wherethe average energy of the beta rays is generally below 2 mev.) theradiative loss of the beta rays is only a small fraction of theirionization loss. Nevertheless, it is possible to produce a useful beamof electromagnetic radiation from a beta emitting radioisotope bydirecting the beta rays through a target of suitable thickness to stop(filter out) most of the beta rays but which will not appreciablydiminish the electromagnetic radiation which is produced in the target;

Referring now to- FIGURE 1, a material 11, some characteristic of whichis to be measured, is placed between a radiation detector 13 such as anionization chamber and a preselected radioisotope 15 (Sr-90. Tl204,etc.) which emits beta particles of a desired average energy. A target17 of suitable composition and thickness is mounted between the betasource 15 and the material being measured so as to prevent beta raysfrom the source 15 from reaching the material 11 without first passingthrough the target 17. As the beta rays from the beta source 15 passthrough the target 17, they will lose energy through ionization andradiation. The target 17 should be of sufi'icient thickness so that mostof the beta rays, preferably over 98 percent, emitted by the beta source15 will be stopped by the target 17. Most of the electromagneticradiation. produced in the target 17, however, will pass out of thetarget because of its comparatively high penetrating power. Practicallyall of the beta rays which pass through the target will be stopped bythe material 11, whereas only a fraction of the electromagneticradiation emitted from the target will be stopped by the material 11.Theelectromagnetic radiation which passes through the material 11 willbombard the ionization chamber 13', developing an ionization currentwhich is dependent on the intensity of the electromagnetic radiation.'

The output of the ionization chamber is fed in the usual manner into anamplifier 19 which amplifies the ionization current, yielding a readingon a meter 2-1. It will be evident that other detectors and indicatingor recording mechanisms may be substituted for the ionization chamber 13and amplifier 19 here shown.

While it is possible to calculate the desired characteristic or propertyof the material 11 from the amount of electromagnetic radiation absorbedby the material 11, this is far from practical in most cases. A moreeconomical procedure is to calibrate the indicating or recording devicein terms of known values of the characteristic for the material 11. Forexample, in a thickness gauge one may calibrate the gauge by insertingvknown thicknesses of the material 11 into the gauge and plotting theradiation intensity, as observed by the readings of the meter 21, as afunction of the thickness of the material 11. The curve thus obtained(not shown) is a function of both the intensity of the beta source 15and the sensitivity of the recording system. A better calibration curveis one such as shown in FIGURE 2 which is obtained by comparing theintensities due to various thicknesses of the material 11 to theintensity without the material 11 or at some arbitrary thickness of thematerial 11 chosen as a standard. This curve is obviously, for a giveninstrument, independent of the source intensity and sensitivity of therecording system, at least as long as these factors do not vary during aseries of measurements. Having obtained such a calibration curve overthe desired range of thicknesses, the intensity recorded on measuringany sample of material 11 will immediately yield its thickness in termsof the standard thickness. Referring now to FIG. 7, there is illustrateda recording system coupled into the measuring system of FIG. 1. The [Onemay, of course, use a] recording meter 23 which will make a permanentrecord on astrip of paper is coupled by shaft 27 to the Contact roller25 [and this strip may be coupled to the gauge] so that the movement ofthe paper 26 corresponds to the movement of the material 11 as it passesthrough the gauge; and the recorded meter deflections 28 on th paperwill thereupon form a permanent record of the thickness of the material11 as it passes through the gauge.

The energy of the electromagnetic radiation produced by electronsthrough radiative collisions with atoms may be of any value up to theenergy of the originating electrons. H. A. Bethe and W. Heitler (Proc.Roy. Soc. A 146, 83, 1934) have given formulas for the probability thatan electron of total energy E ev. in traversing a thickness dt will emita photon of energy between W and W+dW. These formulas are rathercomplicated, and the information contained in them may be grasped morequickly perhaps in graphical than in analytical form. FIGURE 3 shows theenergy distribution of emitted radiation (produced in an aluminumtarget) for various energies of the originating electrons. One canseethat as the energy of the originating electrons increases, there is agreater probability that the electromagnetic radiation will have anenergy which approaches that of the originating electrons.

The effect of the target thickness in the apparatus of FIGURE 1 whenusing Sras the beta source may be seen from FIGURE 4, wherein thethickness of an aluminum target in milligrams per square centimeter isplotted against the thickness of an aluminum absorber required to reducethe total radiation received by the detector to onehalf its value(generally referred to as the half thickness). At relatively smalltarget thicknesses, the betarays from the source 15 predominate andgovern the shape of the curve. As the target thickness increases, moreand more of the beta rays are absorbed until the relative proportion ofelectromagnetic radiation to beta rays becomes large enough to aiiectthe curve. Since the electromagnetic radiation is more penetrating thanthe beta rays, the half thickness of the absorber required to reduce theradiation received by the detector to one-half its value increases untilthe contribution of the beta rays becomes negligible. At a targetthickness sufficient to stop over 98 percent of the original beta rays(greater than 6 half thicknesses for the incident beta rays), the betarays do not materially affect the curve. In a radiation gauge which isconstructed in accordance with the present invention, it is generallydesirable that the detector be affected primarily by the changes inelectromagnetic radiation, the target therefore should preferably have athickness which will stop at least 98 percent of the beta rays whichwould otherwise reach the material being measured.

The eifect of the target material in the apparatus of FIGURE 1 whenusing Sr-90 as the beta source can be seen from FIGURE 5, wherein thereis plotted twice the thickness of the absorber required to reduce theionization current in the detector by a factor of two against the atomicnumber of the absorber for targets of various atomic numbers each of thetarget being sufficiently thick to stop over 98 percent of the beta rayswhich would otherwise reach the absorber. From this graph, it can beseen that an increase in the atomic number of the target results inelectromagnetic radiation of increased average energy, thereby requiringincreased thicknesses of the absorber to reduce the radiation by afactor of two. In addition, it is observed that the stopping power of anabsorber increases rapidly with its atomic number.

Detectors may be used which are particularly effioient for detectingelectromagnetic radiation of particular energies; for example, by aproper choice of ionization chamber. It is well known that as the atomicnumber of the gas used in an ionization chamber is increased, thechamber becomes more efficient in detecting higher energyelectromagnetic radiation. This is illustrated in FIG- URE 6, whereinionization chamber response is plotted against the energy of theelectromagnetic radiation entering the ionization chamber.

It will be evident that person skilled in the art will be able toproduce and detect electromagnetic radiation of predetermined averageenergy level by use of an appropriate beta emitting radioisotope,target, and detector.

A practical thickness gauge Was constructed in accordance with thepresent invention for measuring material havinga weight in excess of 800mg./cm. A beta emitting radioisotope (both Sr-90 and Tl-204 were used)havirtg activities between 50 and 300 millicuries proved adequate forthis gauge. Targets of various materials, including carbon, aluminum,iron, and cop er were used at various times in this gauge. Each targetWas sufliciently thick to stop 98 percent of the beta rays which wouldotherwise reach the material to be measured. The detector used was anionization chamber which was filled at various times with differentgases such as argon, krypton, Xenon, etc. This thickness gauge wassuitable for gauging metals over the following ranges: steel, 0-0.50inch; brass or copper, 041.40 inch; and aluminum, 0-3.0 inches.

As previously mentioned the present invention is applicable to othertypes of industrial nuclear gauges, for example, to gauges which aredesigned for measuring profile, density, composition, etc.Determinations such as composition are possible since the absorption ofelectromagnetic radiation is a function of the atomic number of theabsorber. Accordingly, the invention is not limited to the specificembodiment illustrated, and other uses, modifications, and adaptationswill occur to those skilled in the art. Various features of theinvention believed to be new are set forth in the appended claims.

We claim:

1. In a radiation gauge, wherein one of the properties of a material maybe measured by its absorption of electromagnetic radiation, a betaemitting radioisotope, a material receiving position [in] to permit saidmaterial to pass through said gauge which is spaced from said betaemitter, a target of preselected characteristics positioned between saidbeta emitter and said material receiving position to prevent beta raysfrom said beta emitter from reaching said material receiving positionwithout passing through said target and so as to create a source of electromagnetic radiation which will be directed toward said materialreceiving position, said beta emitting radioisotope and said targetbeing so chosen that the electromagnetic radiation emitted from saidtarget will have a predetermined average energy, [and] a detectorarranged on the side of said material receiving position away from saidbeta emitter and shielded therefrom by said target to detect and measurethe intensity of said electromagnetic radiation not absorbed by [a] thematerial [located at] passing through said material receiving position,and a continuous indicator connected to said detector to continuouslyindicate said property of said material as it passes through said gauge.

2. Apparatus according to claim 1, wherein said target has a thicknesssuflicient to stop at least 98 percent of the beta rays from saidradioisotope which are directed toward and which would otherwise reachthe material receiving position.

3. In a device of the thickness gauge type in which the thickness of amaterial may be measured by its absorption of electromagnetic radiation,a beta emitting radioisotope, a material receiving position [in] topermit said material to pass through said gauge which is spaced fromsaid beta emitter, a target of preselected characteristics positionedbetween said beta emitter and said material receiving position toprevent beta rays from said beta emitter from reaching said materialreceiving position without passing through said target and so as tocreate a source of electromagnetic radiation which Will be directed toward said material receiving position, said beta emitting radioisotopeand said target being so chosen that the electromagnetic radiationemitted from said target will have a predetermined average energy, suchenergy being determined by the absorption characteristics of a material,the thickness of which is to be measured, and a detector arranged on theside of said material receiving position away from said beta emitter andshielded therefrom by said target to detect and measure the intensity ofsaid e1ectro magnet radiation not absorbed by material [located at]passing through said material receiving position, and a continuousindicator connected to said detector to continuously indicate saidmaterial as it passes through said gauge.

4. Apparatus according to claim 3, wherein said target has a thicknesssufiicient to stop at least 98 percent of the beta rays from saidradioisotope which are directed toward and which would otherwise reachthe material receiving position.

5. A method of measuring one of the properties of a passing material bysubjecting [a] said passing material to electromagnetic radiation ofpredetermined average energy which comprises positioning a target ofpreselected characteristics between a beta emitting radioisotope andsaid passing material, wherein a portion of said beta rays emitted bysaid radioisotope in the direction of said material will undergoradiative collisions with the nuclei of said target with resultantemission of electromagnetic radiation, said beta emitting radioisotopeand said target being so chosen that said electromagnetic radiation willhave a predetermined average energy, detecting the electromagneticradiation not absorbed by said material, correlating said dctectedradiation with a property of said material, and continuously indicatingthe property of said material as it passes said detector.

6. A method of measuring one of the properties of a passing material bysubjecting [a] said passing material primarily to electromagneticradiation of predetermined average energy which comprises positioning atarget of preselected characteristics between a beta emittingradioisotope and said passing material, wherein a portion of said betarays emitted by said radioisotope in the direction of said material willundergo radiative collisions With the nuclei of said target Withresultant emission of electromagnetic radiation, said beta emittingradioisotope and said target being so chosen that said electromagneticradiation will have a predetermined average energy, and said targetbeing sufficiently thick to stop at least 98 percent of the beta raysWhich are directed toward and Which would otherwise reach the material,detecting the electromagnetic radiation not absorbed by said material,correlating said detected radiation with a property of said material,and continuously indicating the property of said material as it passessaid detector.

7. A radiation gauge for measuring the properties of a passing material,a beta emitting radioisotope, a target operative to convert betaradiation to electromagnetic radiation, said target positioned betweensaid beta emitting radioisotope and said passing material to direct asource of'electromagnetic radiation toward said material, a detector todetect and measure the intensity of said electromagnetic radiation notabsorbed by the passing material, and a continuous readout meansconnected to said detector to continuously read out said property of thematerial as it passes through the gauge.

8. A radiation gauge for measuring the properties of a passing material,a beta emitting radioisotope, a target operative to convert betaradiation to electromagnetic radiation, said target positioned betweensaid beta emitting radioisotope and said passing material to direct asource of electromagnetic radiation toward said material, a detector todetect and measure the intensity of said electromagnetic radiation notabsorbed by the passing material, a moving strip paper recording meterconnected to said detector, and means for coupling the movement of saidstrip paper to the movement of said passing material to form a permanentrecord of said property of the material as it passes through the gauge.

9. A radiation gauge for measuring the properties of a passing material,a beta emitting radioisotope, a target operative to convert betaradiation to X-rays, said target positioned between said beta emittingradioisotope and said passing material to direct a source of X-raystoward said material, a detector to detect and measure the intensity ofsaid X-rays not absorbed by the passing material, and a continuousreadout means connected to said detector to continuously read out saidproperty of the material as it passes through the gauge.

10. A radiation gauge for measuring the properties of a passingmaterial, a beta emitting radioisotope, a target operative to convertbeta radiation to X-rays, said target positioned between said betaemitting radioisotope and said passing material to direct a source ofX-rays toward said material, a detector to detect and measure theintensity of said'X-rays not absorbed by the passing material, a moyingstrip paper recording meter connected to said detector, and means forcoupling the movement of said strip paper to the movement of saidpassing material to form a permanent record of said property of thematerial as it passes through the gauge.

11. A radiation gauge for measuring the properties of a passingmaterial, a beta emitting radioisotope, a target operative toconvert-beta radiation to electromagnetic radiation, said targetpositioned between said beta emitting radioisotope and said passingmaterial to direct a source of electromagnetic radiation toward saidmaterial, a detector to detect and measure the intensity of saidelectromagnetic radiation not absorbed by the passing material, meansfor correlating said detected radiation as a thickness functionof saidmaterial and a continuous readout means connected to said detector tocontinuously read out said thickness of the material as it passesthrough the gauge.

12. A radiation gauge for measuring the properties of a passingmaterial, a beta emitting radioisotope, a target operative to convertbeta radiation to electromagnetic radiation, said target positionedbetween said beta emitting radioisotope and said passing material todirect a source of electromagnetic radiation toward said material, adetector to detect and measure the intensity of said electromagneticradiation not absorbed by the passing material, means for correlatingsaid detected radiation as a thickness function of said material, amoving strip paper recording meter connected to said detector, and meansfor coupling the movement of said strip paper to the movement of saidmaterial to form a permanent record of said thickness of the material asit passes through the gauge.

13. A radiation gauge for measuring the properties of a passingmaterial, a beta emitting radioisotope, a target operative to convertbeta radiation to X-rays, said target positioned between said betaemitting radioisotope and said passing material to direct a source ofX-rays toward said material, a detector to detect and measure theintensity of said X-rays not absorbed by the passing material, means forcorrelating said detected radiation as a thickness function of saidmaterial and a continuous readout means connected to said detector tocontinuously read out said thickness of the material as it passesthrough the gauge.

14. A radiation gauge for measuring the properties of a passingmaterial, a beta emitting radioisotope, a target operative to convertbeta radiation to X-rays, said target positioned between said betaemitting radioisotope and said passing material to direct a source ofX-rays toward said material, a detector to detect and measure theintensity of said X-rays not absorbed by the passing material, means forcorrelating said detected radiation as a thickness function of saidmaterial, a moving strip paper recording meter connected to saiddetector, and means for coupling the movement of said strip paper to themovement of said material to form a permanent record of said thicknessof the material as it passes through the gauge.

15. In a radiation gauge for measuring a property of a passing material,aradioisotope providing a source of beta radiation having insufiicientenergy to efiect substantial penetration of the thickness of saidmaterial, a target for converting said beta radiation to bremsstrahlunghaving more than suflicient energy to effect substantial penetration ofthe thickness of said material, said radioisotope and said target beingpositioned on one side of said material to direct said bremsstrahlungtoward said material, and means, which includes a bremsstrahlungdetector positioned on the opposite side of said material andsubstantially completely shielded thereby from said beta radiation,responsive to variations in the intensity of said bremsstrahlungpenetrating said material for indicating the variations in said propertyas said material passes said gauge.

16. In a radiation gauge for measuring a property of a passing material,a radioisotope providing a source of beta radiation having insufiicientenergy to efiect substantial penetration of the thickness of saidmaterial, a target for converting said beta radiation to bremsstrahlunghaving a distribution of energies including a major portion havingenergies more than sufiicient to effect substantial penetration of thethickness of said material, said radioisotope and said target beingpositioned on one side of said material to direct said bremsstrahlungtoward said material, and means responsive to lvariations in theintensity of said major portion of said bremsstrahlung for indicatingthe variations in said property as said material passes said gauge, saidintensity variation responsive means including a bremsstrahlung detectorselectively responsive to bremsstrahlung having predetermined energiesin said distribution, whereby the calibration curve for said gaugeattains an optimum shape.

17. In a radiation gauge for measuring a property of a passing material,a radioisotope providing a source of beta radiation having insufiicientenergy to efiect substantial penetration of the thickness of saidmaterial, a target for converting said beta radiation to bremsstrahlunghaving a distribution of energies including a major portion havingenergies more than sufficient to effect substantial penetration of thethickness of said material, said radioisotope and said target beingpositioned on one side of said material to direct said bremsstrahlungtoward said material, and means responsive to variations in theintensity of said major portion of said bremsstrahlung for indicatingthe variations in said property as said material passes said gauge, saidintensity variation responsive means including a bremsstrahlung detectorpositioned on the opposite side of said material and substantiallycompletely shielded thereby from said beta radiation, said detectorbeing selectively responsive to bremsstrahlung having predeterminedenergies in said distribution, whereby the calibration curve for saidgauge attains an optimum shape.

18. A method of measuring one of the properties of a passing material bysubjecting said passing material toelectromagnetic radiation ofpredetermined average energy which comprises positioning a target ofpreselected characteristics in the path of beta rays from a betaemitting radioisotope, whereby a portion of said beta rays emitted bysaid radioisotope in the direction of said target will undergo radiativecollisions with the nuclei of said target with resultant emission ofelectromagnetic radiation, said beta emitting radioisotope and saidtarget being References Cited in the file of this patent or the originalpatent UNITED STATES PATENTS 2,389,403 Arnold Nov. 20, 1945 2,508,772Pontecorvo May 23, 1950 2,629,831 Atchley Feb. 24, 1953 2,675,479Stewart et a1. Apr 13, 1954 2,675,482 Brunton Apr. 13, 1954 2,675,483Leighton et a1. Apr. 13, 1954 2,686,268 Martin et a1 Aug. 10, 19542,797,333 Reiffel June 25, 1957

